The present invention is directed to polymer products, including but not limited to, films, fibers, nonwovens, and sheets, obtained by stretching a composition comprising a biodegradable polyhydroxyalkanoate copolymer. The products exhibit a desirable combination of softness and elasticity while maintaining strength. The products are useful for various biodegradable articles including diaper topsheets, diaper backsheets, garbage bags, food wrap, disposable garments and the like.
Biodegradable polymers and products formed from biodegradable polymers are becoming increasingly important in view of the desire to reduce the volume of solid waste materials generated by consumers each year.
In the past, the biodegradability and physical properties of a variety of polyhydroxyalkanoates have been studied. Polyhydroxyalkanoates are polyester compounds produced by a variety of microorganisms, such as bacteria and algae. While polyhydroxyalkanoates have been of general interest because of their biodegradable nature, their actual use as a plastic material has been hampered by their thermal instability. For example, poly-3-hydroxybutyrate (PHB) is a natural energy-storage product of bacterial and algae, and is present in discrete granules within the cell cytoplasm. PHB is thermoplastic and has a high degree of crystallinity and a well-defined melt temperature of about 180xc2x0 C. Unfortunately, PHB becomes unstable and degrades at elevated temperatures near its melt temperature. Due to this thermal instability, commercial applications of PHB have been extremely limited.
Other polyhydroxyalkanoates, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), have also been investigated. Examples of PHB homopolymer and PHBV copolymers are described in the Holmes et al. U.S. Pat. Nos. 4,393,167 and 4,880,59, and PHBV copolymers are commercially available from Monsanto under the trade name BIOPOL. Unfortunately, polyhydroxyalkanoates such as PHB and PHBV are difficult to process into films for use in various applications. As previously discussed, the thermal instability of PHB makes such processing nearly impossible. Furthermore, the slow crystallization rates and flow properties of PHB and PHBV make film processing difficult. PHBV copolymers are typically produced with valerate contents ranging from about 5 to about 24 mol %. Increasing valerate content decreases the melt temperature of the polymer. However, owing to the relatively small changes in crystallinity, PHBV films often remain stiff and brittle for many applications.
Improved biodegradable copolymers are disclosed by Noda, for example in U.S. Pat. Nos. 5,498,692, 5,536,564, 5,602,227 and 5,685,756. The biodegradable copolymers of Noda comprise at least two randomly repeating monomer units (RRMUs) wherein the first RRMU has the structure [xe2x80x94Oxe2x80x94CH(R1)xe2x80x94(CH2)nxe2x80x94C(O)xe2x80x94] wherein R1is H or C1 or C2 alkyl, and n is 1 or 2, and the second RRMU has the structure [xe2x80x94Oxe2x80x94CH(R2)xe2x80x94CH2xe2x80x94C(O)xe2x80x94] wherein R2 is a C4xe2x80x94C19 alkyl or alkenyl, and wherein at least 50% of the RRMUs have the structure of the first RRMU. These copolymers are advantageous in that they are biodegradable and exhibit a good combination of physical properties which allow their processing into films, sheets, fibers, foams, molded articles, nonwoven fabrics and the like to provide a variety of useful articles. However, these copolymers are not soft and elastic while maintaining strength when they are in their original unstretched state.
Polyhydroxyalkanoate (PHA) copolymers consisting essentially of the repeat units having relatively long alkyl pendant groups of three to nine carbons, such as polyhdroxyoctanoate, are known to exhibit soft and rubber-like elasticity with some level of strength. (See for example, K. D. Gagnon, R. W. Lenz, R. J. Farris, and R. C. Fuller, Macromolecules, vol. 25, pp.3723-3728, 1992.) The utility of soft and elastic products made of such PHA copolymers, however, is severely limited by the disappointingly low melt temperature around 60xc2x0 C. The dimensional stability of the product is compromised even at a temperature of a warehouse in summer which can reach above 80xc2x0 C. Thus, a biodegradable soft and elastic product made of polymers having a higher melt temperature range is desired.
It is often desirable to stretch thermoplastic polymers in order to alter their physical properties. Unfortunately, PHB and PHBV form brittle products that typically break even when drawn to only a very small extent. Various methods have been attempted to improve the stretching processes and the resulting properties of stretched or drawn PHB and PHBV products, for example as disclosed in the Holmes U.S. Pat. No. 4,537,738 and the Barham et al U.S. Pat. No. 4,427,614. Additional methods are disclosed in the Safta European Reference EP 736,563 A1, the Institute of Physical and Chemical Research European Reference EP 849,311 A2, the Waldock WO reference 97/22459, Kusaka et al Pure Appl. Chem., 834(2):319-335 (1998) and Yamamoto et al, Intern. Polymer Processing XII, (1997) 1:29-37. However, such methods have not been particularly successful in providing means for easily forming stretched products having a certain combination of desired physical properties. In particular, the prior art does not provide polymer products having softness and elasticity, while maintaining strength.
Accordingly, it would be advantageous to obtain polymer products which are biodegradable and which have a desirable combination of soft and elastic properties allowing use of the products in a wide range of applications.
Accordingly, it is an object of the present invention to provide new products and methods which overcome disadvantages of the prior art. It is a related object of the present invention to provide polymer products formed of compositions comprising a biodegradable polymer. It is a further object of the invention to provide polymer products which exhibit advantageous combinations of physical properties. It is a more specific object of the invention to provide biodegradable polymer products which exhibit softness and elasticity while maintaining strength. It is another object of the invention to provide methods for easily forming such products. It is yet a further object of the invention to provide articles comprising such polymer products.
These and additional objects and advantages are provided by the products and methods of the present invention. In one embodiment, the invention is directed to polymer products which are obtained by stretching a composition comprising a biodegradable polyhydroxyalkanoate copolymer. The biodegradable polyhydroxyalkanoate copolymer comprises at least two randomly repeating monomer units (RRMUs). The first RRMU has the structure (I): 
wherein R1is H, or C1 or C2 alkyl and n is 1 or 2. The second RRMU is different from the first RRMU and has the structure (II): 
wherein m is from 2 to about 9. At least about 70 mole % of the copolymer comprises RRMUs having the structure of the first RRMU of formula (1). Suitable polymer products include, but are not limited to, films, sheets, fibers, nonwovens, and products formed by bonding a plurality of fibers, for example nonwoven sheets and the like. The polymer products of the invention are advantageous in that they exhibit a combination of softness and elasticity while maintaining strength.
In another embodiment, the invention is directed to methods of forming improved biodegradable polymer products. The methods comprise stretching a composition comprising a biodegradable polyhydroxyalkanoate copolymer at a temperature above the glass transition temperature Tg of the composition and below the melting temperature Tm of the composition. The biodegradable polyhydroxyalkanoate copolymer comprises at least two RRMUs, wherein the first RRMU has the structure of formula (I) and the second RRMU is different from the first RRMU and has the structure of formula (II), all as defined above, with at least about 70 mole % of the copolymer comprising RRMUs having the structure of the first RRMU of formula (1). Conveniently, conventional solid state stretching may be employed. Thus, the methods of the invention comprise relatively easy steps as compared with many of the cumbersome methods of the prior art for producing stretched polymer products, and provide stretched polymer products having an advantageous combination of physical properties.
These and additional objects and advantages will be more fully understood in view of the following detailed description.